Improve tokenizer loading behaviors

Summary:
Ref T4420. Fixes T3309. Two major UX issues here:

  - When the user extends a query ("alin" -> "alinc"), we currently hide all the results, then show them again when the new results arrive. This makes the typeahead feel a bit flickery. Instead, show matching results, then add more results when everything arrives.
  - When loading more results from ondemand sources, we currently do not give you any indication that things are loading. Instead:
    - Show a loading GIF (this might need #design help, @chad).
    - Slightly lighten the control border.
    - I didn't want to do anything like actually add "loading" text because it would cause UI flicker in the 'extend a query' case and some other cases, but otherwise this design is totally made up.

Test Plan: Typed into tokenizers and extended queries, got a better-feeling UI.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T3309, T4420

Differential Revision: https://secure.phabricator.com/D8233